This invention relates to new methods for improving the lattice substitution of heteroatoms in large and extra-large pore borosilicate zeolites. In particular, the aforesaid methods include (1) the deboronation of essentially aluminum free borosilicate zeolites under acid conditions and (2) the reinsertion of heteroatoms in the lattices of deboronated zeolites using aqueous solutions of salts of the corresponding heteroatoms.
Natural and synthetic microporous crystalline molecular sieves including metallosilicates have found widespread industrial applications as catalysts, adsorbents and ion exchangers. These molecular sieves have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species and are similar in size to small organic molecules (roughly 3-15 xc3x85). The adsorptive, catalytic and/or ion exchange properties of each molecular sieve depend largely on its large internal surface area and highly distributed active sites, both of which are accessible through uniform molecularly sized channels and cavities.
According to the Structure Commission of the International Zeolite Association, there are above 120 different microporous crystalline molecular sieve structures. The cage or pore size of these materials is denoted by the number of oxygen atoms (likewise the number of tetrahedral atoms) circumscribing the pore or cavity, e.g., a pore circumscribed by n oxygen-atoms is referred to as an n membered-ring pore, or more simply, n-MR. Molecular sieves containing pores and/or cages with molecular-sized windows (containing 8-MR or larger) can have industrial utility for separations, ion exchange and catalysis. Depending on the largest pore openings that they possess, they are usually categorized into small (8-MR), medium (10-MR), large (12-MR) and extra-large (xe2x89xa714-MR) pore molecular sieves.
The metallosilicates are molecular sieves with a silicate lattice wherein a metal (referred here to as xe2x80x9cheteroelementxe2x80x9d) can be substituted into the tetrahedral positions of the silicate framework. Examples of these metals are boron, aluminum, gallium, iron and mixtures thereof. The substitution of boron, aluminum, gallium and iron for silicon results in an imbalance in charge between the silicon and the corresponding trivalent ions in the framework. In turn, such a change in the framework charge alters the ion exchange capacity of a material as well as the adsorptive and catalytic behavior because of the distinct physicochemical properties of these heteroelements. Thus, the utility of a particular molecular sieve in a particular adsorptive, catalytic or ion exchange application depends largely not only on its crystal structure but also on its properties related to the framework compositions. For example, stronger acid strength in zeolite catalysts is required for iso-butane/butene alkylation at lower reaction temperatures to simultaneously achieve higher activity and lower deactivation rate of the catalyst. By contrast, as demonstrated by S. Namba et al. (Zeolites 11, 1991, p.59) in studies on the alkylation of ethylbenzene with ethanol over a series of metallosilicates with MFI (ZSM-5) zeolite structure, namely, B-ZSM-5, Sb-ZSM-5, Al-ZSM-5, Ga-ZSM-5 and Fe-ZSM-5, the para-selectivity to para-diethylbenzene is largely related to the acid strength of the catalysts and the weaker acid sites provide a higher para-selectivity.
In nature, molecular sieves commonly form as geothermally heated ground water passes through silicate volcanic ash. Early attempts to synthesize zeolites centered around recreating the high-pressure, high-temperature conditions found in nature. Barrer (J. Chem. Soc., 1948, p.127) demonstrated the first successful zeolite synthesis (mordenite) while Milton (U.S. Pat. No. 2,882,243 (1959)) developed the large-scale zeolite synthesis at low temperatures and pressures that allowed zeolites to gain industrial importance. These zeolite syntheses relied on the presence of alkali metal cations in the synthesis mixture to serve as a mineralizing agent. The alkali metal cations also play a role in the structure direction of the particular zeolite that forms.
Building on the concept of cationic structure direction, the range of cations was subsequently expanded later on from the inorganic metal cations to organic cations such as quaternized amines. In the recent years, the use of organic molecules to direct the formation of zeolites and other molecular sieves has become commonplace and given rise to an increasing number of novel molecular sieves, leading to breakthroughs in molecular sieve science and providing an impetus in developing new process chemistry.
As mentioned before, today over 120 molecular sieve structures have been discovered. Some of them counterparts to the naturally occurring molecular sieves, whereas others have no natural analog. Theoretical studies of molecular sieve structures and structure types indicate that only a small fraction of the configurations possible for microporous, crystalline molecular sieves have been discovered. Apparently, the major roadblock in tailoring and utilizing molecular sieve materials for specific applications in catalysis, adsorption and ion exchange is the development of synthesis methods to produce the desirable structure with the desirable framework composition.
In the principle, there are two routes leading to the formation of a particular molecular sieve structure with a particular framework composition, e.g., a particular metallosilicate such as aluminosilicate, gallosilicate, ferrosilicate or borosilicate of the same crystal structure: (1) direct synthesis and (2) post-synthetic treatment (secondary synthesis). These two routes will be discussed next.
The direct synthesis is the primary route of the synthesis of molecular sieves. The major variables that have a predominant influence on the molecular sieve structure crystallized include: the gross composition of the synthesis mixture, temperature and time. Even though each variable contributes to a specific aspect of the nucleation and crystallization of the molecular sieves, there is substantial interplay between these elements during the formation of molecular sieves. In the presence of heteroelement X (X=Al, Ga, Fe or B, for example, or X=none for pure-silica molecular sieves), the Si/X ratio will determine the elemental framework composition of the crystalline product; but the amount of the heteroelement in the synthesis mixture also can determine which structure, if any, crystallizes. In addition to the Si/X ratio, various other factors related to the gross composition of the synthesis mixture also play an important role. These factors include: OHxe2x88x92 (or Fxe2x88x92) concentration, cations (both organic and inorganic), anions other than OHxe2x88x92 (or Fxe2x88x92), and water concentration. There are also history-dependent factors such as digestion or aging period, stirring, nature (either physical or chemical) of the synthesis mixture, and order of mixing. In short, depending on the nature of the molecular sieves and the chemistry of their formation, some of these molecular sieve structures can be synthesized in a broad spectrum of framework compositions such as ZSM-5 containing none heteroatoms (Si-ZSM-5 or silicalite-1), Al (Al-ZSM-5), B (B-ZSM-5), Fe (Fe-ZSM-5) and Ga (Ga-ZSM-5), whereas the synthesis of other structures succeeds only if certain heteroatom is present in the synthesis mixture and, in turn, incorporated into the framework, or some structures containing specific heteroatom(s) can be synthesized only in a limited range of Si/X ratio, or some structures containing specific heteroatom(s) can be synthesized only if certain specific, usually more expensive, structure-directing agents are employed. These complicated relationships between zeolite structures, framework compositions and structure-directing agents have been discussed in many publications and patents, e.g., by Zones et al. in J. Am. Chem. Soc. 122, 2000, p.263.
For example, U.S. Pat. No. 4,963,337 (xe2x80x9cthe ""337 patentxe2x80x9d) to Zones claims a procedure to synthesize borosilicate zeolite SSZ-33 (which is the first synthetic zeolite containing intersecting 10- and 12-membered ring channels) by using N,N,N-trimetyl-8-tricyclo[5.2.1.02,6]decane ammonium cations as a structure-directing agent. The direct synthesis of aluminosilicate, gallosilicate and ferrosilicate SSZ-33 using this structure-directing agent is up to date not successful.
U.S. Pat. No. 4,910,006 to Zones et al. claims also a procedure to synthesize aluminosilicate zeolite SSZ-26 (which has a very similar crystalline structure to SSZ-33) using N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2-hexamethyl[4.3.3.0]propellane-8,11-diammonium cations as a structure-directing agent. However, this structure-directing agent is difficult to make and, hence, much more expensive than N,N,N-trimetyl-8-tricyclo[5.2.1.02,6]decane ammonium cations which is used for the synthesis of borosilicate SSZ-33. Therefore, a new way to prepare aluminosilicate SSZ-33 is desired.
In addition to the preparation of a specific molecular sieve structure with a specific framework composition via the aforesaid direct synthesis, post-synthetic treatments (or secondary synthesis) often provide a more economic alternative route to achieve this goal. The post-synthetic treatment techniques all operate on the same principle: the desirable heteroatoms such as Al, Ga and Fe are inserted into lattice sites previously occupied by other T-atoms such as B. For example, in the approach in the ""337 patent of making borosilicate SSZ-33 (referred to as B-SSZ-33) into aluminosilicate SSZ-33 (referred to as Al-SSZ-33) with much stronger framework acid sites, it is taught to heat a calcined B-SSZ-33 in an aqueous Al(NO3)3 solution at xcx9c100xc2x0 C. The result is that an Al-SSZ-33 product was obtained with considerably enhanced acidity. Example 9 of the ""337 patent for the Constraint Index determination demonstrates that the total feed conversion at 800xc2x0 F. over this resulting Al-SSZ-33 is 62% for the acid-catalyzed n-hexane/3-methylpentane cracking. By contrast, but as expected from the low acidity associated with boron-atoms in B-SSZ-33 framework, Example 8 of the ""337 patent shows that the B-SSZ-33 basically has no activity for the same reaction under the same conditions. Clearly, these two examples demonstrate the benefit of making catalytically more active aluminosilicate zeolites from their borosilicate counterparts via post-synthetic treatments. Furthermore, the present invention teaches a superior method for measurably introducing heteroatoms into zeolite structures formerly occupied by boron atoms. This improvement is contrasted with some of our own prior art disclosed in the ""337 patent. There is given a variety of experimental evidences demonstrating the effectiveness of our new invention.
In summary, direct synthesis is often difficult or impossible for preparing some useful structures of catalytically active alumino-, gallo- or ferrosilicate zeolites. As shown in e.g., the ""337 patent, it is possible to synthesis novel borosilicate zeolites structures. Borosilicate zeolites, however, are not sufficiently catalytically active to be practicable for certain hydrocarbon conversion processes.
It would be advantageous to have a method of replacing the boron in borosilicate zeolites with precise amounts of a preferred heteroatom for enhancing and controlling the catalytic activity of the resultant zeolite. The present invention provides such a method.
The present invention describes improved methods to prepare large and extra-large pore aluminosilicate, gallosilicate and ferrosilicate zeolites via post-synthetic treatments of large and extra-large pore essentially aluminum free borosilicate zeolites as starting materials.
Accordingly, in one embodiment, it is an object of the present invention to provide an 2-step method for preparing crystalline zeolites by (a) contacting a calcined large or extra-large pore essentially aluminum free borosilicate zeolite with an acid (e.g., aqueous HCl solution), thereby producing an at least partially deboronated zeolite; (b) contacting said at least partially deboronated zeolite with a solution selected from the group consisting of an aqueous aluminum salt solution, thereby producing an aluminosilicate zeolite; an aqueous gallium salt solution, thereby producing a gallosilicate zeolite; an aqueous iron salt solution, thereby producing a ferrosilicate zeolite; and mixtures thereof; and (c) where the contacting in step (b) occurs at a pH of not greater than about 3.5.
In another embodiment, it is an object of the present invention to provide an 1-step method for preparing crystalline zeolites by contacting a calcined large or extra-large pore essentially aluminum free borosilicate zeolite with a solution selected from the group consisting of an aqueous aluminum salt solution, thereby producing an aluminosilicate zeolite; an aqueous gallium salt solution, thereby producing a gallosilicate zeolite; an aqueous iron salt solution, thereby producing a ferrosilicate zeolite; and mixtures thereof; and where the contacting occurs at a pH of not greater than about 3.5.
Among other factors the present invention provides methods for making heteroatom lattice substituted zeolites and catalysts having precisely controlled acidities and thus very finely controlled catalytic properties. These methods allow the xe2x80x98fine-tuningxe2x80x99 of the resultant catalysts to have the desired catalytic activity.
A. Processs Overview
We have found that large and extra-large pore aluminosilicate/gallosilicate/ferrosilicate zeolites can be prepared by using their essentially aluminum free borosilicate counterparts as starting materials. The method includes, but is not limited to, using the borosilicate counterparts of the following zeolites as a starting material for the present invention: SSZ-24, SSZ-31, SSZ-33, SSZ-41, SSZ-42, SSZ-43, SSZ-45, SSZ-47, SSZ-48, SSZ-53, SSZ-55, SSZ-59, SSZ-64, CIT-1, CIT-5 and UTD-1. U.S. Pat. Nos. 4,834,958/4,936,977 (SSZ-24), U.S. Pat. No. 5,106,801 (SSZ-31), U.S. Pat. No. 4,963,337 (SSZ-33), U.S. Pat. Nos. 5,653,956/5,770,175 (SSZ-42), U.S. Pat. No. 5,965,104 (SSZ-43), U.S. Pat. No. 6,033,643 (SSZ-45), U.S. Pat. No. 6,156,260 (SSZ-47), U.S. Pat. No. 6,080,382 (SSZ-48), U.S. Pat. No. 6,464,956 (SSZ-59), U.S. Pat. No. 5,512,267 (CIT-1), U.S. Pat. No. 6,040,258 (CIT-5) and U.S. Pat. No. 5,489,424 (UTD-1), and pending application U.S. Ser. No. 09/836,923 (SSZ-53), U.S. Ser. No. 09/520,640 (SSZ-55), U.S. Ser. No. 10/211,890 (SSZ-64), teaching the synthesis of SSZ-24, SSZ-31, SSZ-33, SSZ-41, SSZ-42, SSZ-43, SSZ-45, SSZ-47, SSZ-48, SSZ-53, SSZ-55, SSZ-59, SSZ-64, CIT-1, CIT-5 and UTD-1, respectively, are incorporated herein by reference in their entireties.
It can be important that the borosilicate zeolite be essentially aluminum free. By xe2x80x9cessentially aluminum freexe2x80x9d we mean that the borosilicate zeolite contains less than 500 wt. ppm aluminum, preferably less than 300 wt. ppm aluminum, more preferably less than 100 wt. ppm, yet more preferably less than 50 wt. ppm, still more preferably less than 25 wt. ppm, and most preferably less than 10 wt. ppm aluminum.
Being essentially aluminum free allows the borosilicate zeolite to be modified effectively to make a heteroatom lattice substituted zeolite having a precisely controlled acidity thus resulting in very well controlled catalytic properties.
The examples in the present application show how the catalytic properties can be precisely tailored by using an essentially aluminium free borosilicate starting material.
The importance of the starting materials being essentially aluminum free is significant. Even relatively small amounts of aluminum in the zeolite can make it difficult to control the acidity and the catalytic activity of the finished catalyst material. The use of aluminum free starting materials is essential to ensure an aluminum free borosilicate zeolite. Use of an essentially aluminum free silica source such as Cabosil M-5 is important to produce an essentially aluminum free borosilicate zeolite. The aluminum specification of Cabosil M-5 is less than 4 ppm. Cabosil M-5 is used in Example 1 of the present application. Other starting materials can also cause the introduction of aluminum into the framework, thus all starting materials and equipment should be essentially aluminum free in order to avoid contamination of the borosilicate zeolite with aluminum and inadvertent introduction of aluminum (and thus acidity) into the borosilicate zeolite produced.
Two embodiments of the method of the invention are described below.
B. Two-Step Methodxe2x80x94Method A
One method for making large or extra-large pore alumino-, gallo- and ferrosilicate zeolites according to the invention includes contacting a calcined large or extra-large pore essentially aluminum free borosilicate zeolite with an acid (e.g., 0.01 N aqueous HCl solution), thereby producing an at least partially deboronated zeolite. The next step is contacting the at least partially deboronated zeolite with a solution selected from the group consisting of an aqueous aluminum salt solution, thereby producing an aluminosilicate zeolite; an aqueous gallium salt solution, thereby producing a gallosilicate zeolite; an aqueous iron salt solution, thereby producing a ferrosilicate zeolite; and mixtures thereof. The solubilized aluminum salt preferably includes aqueous Al(NO3)3 and/or Al2(SO4)3 solution. The solubilized gallium salt preferably includes Ga(NO3)3 and/or Ga2(SO4)3. The solubilized iron salt preferably includes Fe(NO3)3 and/or Fe2(SO4)3.
The second contacting step occurs at a pH of not greater than about 3.5. Both contacting steps occur at a temperature of from about ambient temperature to about 300xc2x0 C. Pressure is from about 0 to about 1000 psig, preferably ambient. Both contacting steps preferably occur under stirring or tumbling.
In the second contacting step, the solution is an aqueous solution consisting of aluminum salt or gallium salt or iron salt or mixture thereof and wherein the weight ratio of the at least partially deboronated zeolite to said salt is from about 1:0.01 to about 1:100. In the second contacting step, the water content is from about 50 weight percent to about 99.5 weight percent of the solution.
C. One-Step Methodxe2x80x94Method B
Another embodiment is a one-step method for making large or extra-large pore alumino-, gallo- and ferrosilicate zeolites including contacting a calcined large or extra-large pore essentially aluminum free borosilicate zeolite with a solution selected from the group consisting of an aqueous aluminum salt solution, thereby producing an aluminosilicate zeolite; an aqueous gallium salt solution, thereby producing a gallosilicate zeolite; an aqueous iron salt solution, thereby producing a ferrosilicate zeolite; and mixtures thereof; and wherein said contacting occurs at a pH of not greater than about 3.5. The contacting step occurs at a temperature of from about ambient temperature to about 200xc2x0 C. The same conditions and limitations apply as in the two-step method (Method A) described above.
The invention will be further clarified by the following Illustrative Embodiments, which are intended to be purely exemplary of the invention. The indexes for the Examples, Tables and Figures are shown below.